Primary myelofibrosis (PMF) is regarded as a chronic myeloproliferative disorder. It is characterized by marrow fibrosis, leukoerythroblastosis, tear drop erythrocytes and extramedullary hematopoiesis. Most patients are in their late 50s when first diagnosed. Pediatric PMF is said to be quite rare. Here describe a female infant with PMF. The patient was born on Aug. 7, 1991. The pregnancy and delivery were uneventful. Hepatomegaly was noted soon after birth. Combined blood counts showed polycythemia and leukocytosis. It was thought to be extramedullary hematopoiesis due to intrauterine infection. She was followed up in another hospital, but since her condition was unchanged she was admitted to our hospital for further medical examinations at age 7 months. On the peripheral blood smear, there were tear drop erythrocytes, normoblasts and early myeloid elements. Repeated bone marrow aspirations were dry taps. This case presented the classical findings of fibrosis of the bone marrow on bone marrow biopsy. She is in good health without any therapy until now. A review of 7 cases of PMF, including our case, in Japanese children was made and discussed in comparison to adult cases.